Pretreatment of rats with an agent that inhibits uptake into serotoninergic neurons [Lilly 110140: 3-(p-trifluoromethylphenoxy)-N-methyl-3-phenylpropylamine hydrochloride] prevented the depletion of brain serotonin by 4-chloroamphetamine, presumably by preventing the entry of 4-chloroamphetamine into the serotonin neuron. When the uptake inhibitor was given after 4-chloroamphetamine, the lowering of both serotonin and tryptophan hydroxylase levels in brain was reversed. Serotonin levels returned to normal after 110140 administration to 4-chloroamphetamine-treated rats at a rate similar to the calculated rate of serotonin turnover in rats treated with 110140 alone. Progressively less reversibility of the 4-chloroamphetamine effect occurred when 110140 was injected at 8, 16 and 24 hr after 4-chloroamphetamine, and no reversibility was observed when 110140 was injected at 32 or 48 hr after 4-chloroamphetamine. These findings indicate that the depletion of brain serotonin by 4-chloroamphetamine is initially reversible but that there is a gradual transition into an irreversible effect between 24 and 48 hr after 4-chloroamphetamine injection. Apparently the prolonged presence of 4-chloroamphetamine inside the serotonin neuron produced by continual reuptake of 4-chloroamphetamine is required for the semi-permanent depletion of brain serotonin stores.